1. Field of the Invention
The present invention relates to an ac-powered LED light engine to gear up and down the number and current of excited LED sub-arrays in accordance with the voltage level of the rectified sinusoidal input voltage.
2. Description of the Prior Art
LED-based lighting devices are gradually becoming the preferred lighting equipment because of having a relatively longer lifetime to reduce maintaining cost, and being less likely to get damaged.
Technically, LEDs need to be DC-driven. So, an AC sinusoidal input voltage would normally be rectified by a full-wave or a half-wave rectifier into a rectified sinusoidal input voltage before coming into use. In the vicinity of the beginning and end of each DC pulse cycle (aka “dead time”) where the input voltage is less than the combined forward voltage drop of the LEDs, the LEDs cannot be forward-biased to light up. The dead time in union with the conduction angle constitutes a full period of the rectified sinusoidal input voltage. A longer dead time translates to a smaller conduction angle, and hence a lower power factor because the line current is getting too thin to be similar in shape to the line voltage. Traditional LED drivers usually come along with three application problems.
The first problem would be the need for a more complicated and more expensive driving circuit consisting of a filter, a rectifier, a power factor corrector (PFC), etc. to drive LEDs. The short-life electrolytic capacitor used as an energy-storage component in the PFC is the key reason accounting for the shortened overall lifespan of the whole LED illuminating apparatus, cancelling out the virtues of LED lighting.
The second problem would be the flicker phenomenon due to no current flow through the LEDs during the dead time. The LEDs would immediately light up with a positive driving current, and immediately go out with a zero driving current, causing the LEDs to flicker if there exists a dead time. The flicker phenomenon takes place during the dead time at a repetition rate of twice the AC sinusoidal frequency.
The third problem would be a relatively lower power factor exhibited by a low-power PFC with a loop current too weak to be precisely sensed to correctly shape the AC input current into a sinusoidal waveform. The power factor is used to measure the electricity utilization. The more similar the line current is to the line voltage, the better the electricity utilization and the higher the power factor. When the line current and the line voltage are consistent in terms of identical phase and identical shape, the power factor would reach its maximum value of 1.
The conventional PFC needs to sense its loop current for the purpose of aligning the line current with the line voltage. If the loop current appears too low to be precisely sensed by the current-sensing circuitry in the PFC stage, the PFC would fail to properly keep the line current in phase and in shape with the line voltage to achieve a high power factor. Often mentioned in the same breath with the issue of a low PF is the issue of a high total harmonic distortion (THD). According to the theory of Fourier series expansion of any periodic signal, any discontinuous or jumping points in the periodic waveform would incur higher-order harmonics on top of the fundamental component, causing the THD to increase. The THD resulting from the discontinuous or jumping points in the AC input current waveform would have much to do with the existence of the dead time.
Simplifying the electronic circuit, reducing the manufacturing and maintaining costs, eliminating the flicker phenomenon, as well as improving the power factor still remain the main topics put at the top of the agenda when it comes to developing new-generation LED lighting apparatuses.